Cherie Blair and her son Euan are under investigation for potentially breaching planning rules, it has emerged.

Westminster council has confirmed it is looking into claims that a property owned by the wife and son of former prime minister Tony Blair has been divided into two homes without permission.

A neighbour complained that the basement of Euan's £1.3m central London apartment was being sub-let and had a separate letterbox and flat number, according to the Telegraph.

The mother and son's previous attempt to secure permission to create two dwellings was rejected, it said.

A Westminster council spokeswoman said: "The council has begun an investigation following a complaint of unauthorised change of use at this property."

The Blair family is "confident" the council's investigation will find everything is in order, its spokeswoman said on Saturday.

The day the sale went through in May 2010 Westminster council received a planning application to allow steps to be built down to the basement from the street as well as the removal of an internal stairway, but that was amended after objections from neighbours, the Telegraph reported.

The new plans, which provided for a basement guest room, toilet and kitchen, were passed but with a warning that "permission does not authorise the use of the basement as a separate self-contained flat", it added.

A spokeswoman for the Blairs said: "The Blairs are happy for the council to check, they're confident that everything will be in order."